High power amplifiers are often designed by combining active devices in parallel. A number of devices combined in parallel behave electrically similar to one larger device and can be used in a similar manner. A major difficulty with existing techniques for combining devices in parallel is that they increase only the maximum operating current, not the voltage.
A further disadvantage with existing designs is that the output impedance (both the actual impedance seen looking into the output, and the impedance required for maximum power or efficiency) is related to the voltage and current such that it decreases for large numbers of devices combined in parallel. For low frequency applications, this is not a problem, but for RF applications where it is mandatory to drive a load on the order of 50 ohms, it is an important consideration, as low loss matching networks that can transform low impedances to 50 ohms are difficult to realize. Compounding this problem is the fact that higher frequency devices need smaller geometries (for example, base width, or channel length) to achieve high gain. These small geometries tend to reduce device breakdown voltage, so for a given output power, the output impedance decreases even further.
One existing method of overcoming these problems is to connect the devices in series, or to make a combination of series and parallel connections. Specifically, the devices are placed so that the Emitter-Collector (or Source Drain) junctions are in series, sharing the power supply voltage. This technique has been described in "Amplifying Devices and Low-pass Amplifier Design", E. Cherry, D. Hooper, published by John Wiley and Sons, Inc., New York, and has been used to a small extent at lower frequencies. In the same publication, totem pole amplifiers, or bean-stalk amplifiers have been described, but it appears that practical implementations based upon this publication have not been realized. Totem-pole amplifiers for use at microwave frequencies have been described in "A New Power Amplifier Topology and Series Biasing and Power Combining of Transistors", M. Shifrin, Y. Ayasli, P. Katzin, in the proceedings of the 1992 Microwave and mm-wave Monolithics Symposium.
Another widely used attempt to solve these problems is the "Cascode" configuration in which a "top" device is driven only through the "common" terminal. This has been described in "Electronic Devices and Circuit Theory" by R. Boylestad and L. Natelsky, Prentice-Hall Inc., 1982, pages 334-335.
A problem with some of the existing stacked amplifiers referred to above lies in the drive of the base (or gate) of certain of the amplifiers in the stack. The drive of the bottom device is not a problem as an input signal is connected directly to the device. The next device up is more difficult to drive because the drive voltage must be referenced to the emitter (or source) of that device, but this electrode already has a high RF swing on it due to its connection to the bottom device. The next device is even more difficult to drive.